From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([66.187.233.31]:20691 "EHLO mx1.redhat.com") by vger.kernel.org with ESMTP id S268753AbUHLUb1 (ORCPT ); Thu, 12 Aug 2004 16:31:27 -0400 Date: Thu, 12 Aug 2004 13:30:24 -0700 From: "David S. Miller" Subject: Re: clear_user_highpage() Message-Id: <20040812133024.36e5b7ee.davem@redhat.com> In-Reply-To: <20040812220025.27cb260a.ak@suse.de> References: <20040811161537.5e24c2b6.davem@redhat.com> <20040811165307.46ff1eb6.davem@redhat.com> <20040812020825.GA14411@wotan.suse.de> <20040811194545.0034428b.davem@redhat.com> <20040812110924.0713f5d9.ak@suse.de> <20040812125059.298ae914.davem@redhat.com> <20040812220025.27cb260a.ak@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit To: Andi Kleen Cc: torvalds@osdl.org, linux-arch@vger.kernel.org List-ID: On Thu, 12 Aug 2004 22:00:25 +0200 Andi Kleen wrote: > Well, the writes are usually faster. While they don't use the > cache they use special write combining buffers in the CPU > that hold the data until it can blast out a full cache. Advantage > is that it doesn't have to read anything first. Sure. Sparc64 has this two, in fact is has a full 2K write cache to absorb all of the cpu's write traffic. > How effective this is depends on the CPU, in general newer > x86s tend to have much larger WC buffers than the previous > generation (e.g. Intel just enlarged them again in Prescott) > > Unlike all other stores on x86 they are also very lazily ordered > and need explicit memory barriers. The cache-bypassing 64-byte block stores behave this way on sparc64. > This still has the same problem: in the end the data > is out of cache and when someone else needs it later they eat > large penalties. If it was in the cache to begin with, it will stay there. This is the case the x86_64 bits lose for, they'll kick the lines out. If it is out of cache, no L2 cache lines are allocated. This is how x86_64 will perform. I think the "hit" case behavior difference could make a difference.